Inverse Size Scaling of the Nucleolus by a Concentration-Dependent Phase Transition

Just as organ size typically increases with body size, the size of intracellular structures changes as cells grow and divide. Indeed, many organelles, such as the nucleus [1, 2], mitochondria [3], mitotic spindle [4, 5], and centrosome [6], exhibit size scaling, a phenomenon in which organelle size depends linearly on cell size. However, the mechanisms of organelle size scaling remain unclear. Here, we show that the size of the nucleolus, a membraneless organelle important for cell-size homeostasis [7], is coupled to cell size by an intracellular phase transition. We find that nucleolar size directly scales with cell size in early C. elegans embryos. Surprisingly, however, when embryo size is altered, we observe inverse scaling: nucleolar size increases in small cells and decreases in large cells. We demonstrate that this seemingly contradictory result arises from maternal loading of a fixed number rather than a fixed concentration of nucleolar components, which condense into nucleoli only above a threshold concentration. Our results suggest that the physics of phase transitions can dictate whether an organelle assembles, and, if so, its size, providing a mechanistic link between organelle assembly and cell size. Since the nucleolus is known to play a key role in cell growth, this biophysical readout of cell size could provide a novel feedback mechanism for growth control. [Display omitted] •Nucleolar size scales directly with cell size during development•Nucleolar size scales inversely with cell size across RNAi conditions•Nucleolar size and assembly depend on the concentration of components•Direct and inverse scaling regimes can be explained by a phase transition model Weber and Brangwynne show that nucleolar size is determined by the concentration of maternally loaded components, which condense into nucleoli only above a threshold concentration. This work suggests that intracellular phase transitions may provide a general mechanism for organelle assembly that inherently couples organelle size with cell size.